Field of the Invention
The invention relates to methods and devices for equalizing and decoding an error-protection-coded data signal transmitted via a radio channel.
In the transmission of data via a radio channel, in particular a mobile radio channel, difficulties arise in particular which are associated with the fact that the transmission properties of the radio channel change continuously with time, and that a received radio signal suffers from interference from radio signals of other mobile radio subscribers.
Adaptive equalizers are used at the receiver end in order to take account of the continuous changes in the transmission properties of the radio channel. Adaptive equalizers are distinguished in that they are equipped with what is termed a channel estimator. The channel estimator continuously determines the current transmission properties (pulse responses) of the radio channel and communicates them to the equalizer. The equalizer is thereby put into the position of carrying out an equalization of the obtained received signal that is “adaptive” (that is to say adapted to the instantaneous state of the radio channel).
Coding/decoding of the transmitted/received data signal is used for the purpose of effective error protection of the transmitted data. In the case of the coding at the transmitter end, redundancy is added to the output data signal and enables the receiver to identify and correct detection errors occurring during data detection. The effect of interference can thereby be reduced.
The article titled “Combined Turbo Equalization and Turbo Decoding” by D. Raphaeli and Y. Zarai, IEEE Communications Letters, Vol. 2, No. 4, 1998, pages 107 and 109 describes an iterative receiver structure which contains a maximum a-posteriori (MAP) symbol estimator for adaptive channel estimating, and a downstream turbo decoder for decoding. The digital signal output by the MAP symbol estimator is present in what is termed a log-likelihood ratio (LLR) format. By calculating the variance of the channel noise, it is converted at the input of the turbo decoder into a “soft” (that is to say value-continuous) input signal required by the decoder.
U.S. Pat. No. 5,214,675 describes a configuration containing a linear equalizer with variable equalizer coefficients and a channel decoder. Disposed upstream of the channel decoder is an arithmetic unit in which the channel strength and the variance of the channel noise are calculated. The calculated values are supplied to the channel decoder.